rkt, often written as Rocket, is the closest competing application containerization platform from CoreOS that was started as a more secure application container runtime. Over time, Docker has closed a number of its security failings but unlike rkt, which runs with limited privileges as a user service, Docker's main service runs as root. This means that if someone manages to break out of the Docker container, they will automatically have full access to the host's root, which is obviously a really bad thing from an operations perspective while with rkt, the hacker would also need to escalate their privilege from the limited user. While this comparison here isn't painting Docker in great light from a security standpoint, if its development trajectory is to be extrapolated, it is possible and likely that this issue will be heavily mitigated and/or fixed in the future.

Another interesting difference is that unlike Docker, which is designed to run a single process within the container, rkt can run multiple processes within a container. This makes deploying multiple services within a single container much easier. Now, having said that, you actually can run multiple processes within a Docker container (we will cover this at a later point in the book) but it is a great pain to set that up properly but I did find in practice that the pressure to keep services and containers based on a single process really pushes the developer to create containers as true microservices instead of treating them as mini VMs so don't consider this necessarily as a problem.

While there are many other smaller reasons to choose Docker over rkt and vice versa, one massive thing cannot be ignored: the rate of adoption. While rkt is a bit younger, Docker has been adopted by almost all big tech giants, and there doesn't seem to be any sign of stopping the trend. With this in mind, if you need to work on microservices today, the choice is probably very clear but as with any tech field, the ecosystem may look much differently in a year or even just a couple of months.